1. Field of the Invention
The present invention pertains to a method and system for directional drilling of wellbores wherein the wellbore deviates from a substantially vertical portion of the wellbore to a substantially horizontal portion through a radius in the range of approximately 200 feet to 400 feet or a so-called build curvature of approximately 15.degree. to 25.degree. per 100 feet of wellbore length.
2. Background
A large number of hydrocarbon containing earth formations exist in various parts of the world which have a vertical thickness of about 300 feet to 400 feet or more. Many of these reservoirs are of a relatively low permeability type rock, such as limestone, and may have a substantial number of spaced apart natural vertical fractures. These types of formations or reservoirs are more likely to be economically produced if the wellbore is formed to extend generally horizontally through the formation to increase the amount of hole "depth" within the formation itself. Accordingly, forming such wellbores desirably involves drilling a vertical portion of the wellbore extending downward from the surface then curving the wellbore into a relatively highly deviated or near horizontal direction within and through the formation itself. It is also generally desirable that the radius of the deviated section of the wellbore which extends from the vertical to the horizontal portion be in the range of about 200 feet to 400 feet. In this way drilling may take place to identify the formation thickness, the wellbore may be plugged back to the top of the production zone or blocked by a whipstock of the like and then redrilled to form the transition portion from the vertical to the near horizontal. The curved and horizontally extending wellbore portions should be left in an open hole condition, if possible, to maximize wellbore length available for production of mineral values.
Unfortunately, up to the time of the development of the present invention, known techniques for drilling highly deviated or generally horizontal wellbores fall into categories which are rather extreme with respect to the desired wellbore configurations for producing the types of formations mentioned. So called conventional deviated drilling techniques for transforming the wellbore from a vertical to generally horizontal direction use conventional rotary drilling equipment and methods wherein the radius of curvature of the drillstem generally cannot be reduced to less than about 1000 feet to 1200 feet and may range upward to a radius of 3000 feet. Such drilling techniques may make it impossible to drill cost effective wells into productive zones having a thickness in the ranges abovementioned.
The other technique used for drilling generally horizontal wellbores is sometimes referred to as drainhole drilling wherein deviation of the wellbore from the vertical to horizontal is through a rather small radius or high build curvature. High curvature drilling to provide drainholes and the like typically is carried out with a curvature radius of about 30 feet which produces a so-called wellbore angular build rate in the range of about 200.degree. per 100 foot of wellbore length. The total length of horizontal or deviated hole that may be produced by such a technique is typically in the range of about 100 feet to 500 feet. The drilling equipment is required to be very specialized and, accordingly, the cost per unit length of horizontal or deviated hole is relatively high.
One rather important consideration in high curvature drilling techniques is the lack of control of the direction of the horizontal portion of the borehole. The high angular build rate is not conducive, with known equipment, to good directional control and the prospect of equipment failure makes this type of curved or deviated hole drilling relatively unattractive.
Accordingly, considering the type and thickness of many known mineral value reservoirs which may be produced, there has been a continuing need to develop deviated or directional drilling methods which will provide the medium curvature geometry of the wellbore desired and which will overcome the disadvantages of conventional deviated hole drilling and so-called high curvature horizontal or drainhole type drilling techniques. It is to this end that the present invention has been developed with the discovery and development of a unique method and an improved drillstem system for drilling medium curvature wellbores with particular but not exclusive emphasis on wellbores drilled with curvatures in the range of approximately 15.degree. to 25.degree. per 100 feet of wellbore length or a wellbore radius of about 200 feet to 400 feet.